1. Field of the Invention
The present invention relates to an optical switch to be used for setting and switching optical paths in an optical communication system or the like. Particularly, the present invention relates to an optical switch and its assembling method, in which optical paths are switched by changing the conditions of reflection and transmission of light as a result of moving a liquid in a groove (i.e., a gap) formed at some point of the optical path.
2. Description of the Related Art
Heretofore, optical switches for switching optical paths by moving a liquid and methods for manufacturing these optical switches have been proposed The optical switch comprises optical waveguides which are formed in a substrate and cross each other, a groove having a wall surface at a predetermined angle crossed from the optical axis of optical waveguide positioned at the intersection of the optical waveguides, and a refractive index-matching liquid being sealed in the groove. If the groove is empty or is filled with a liquid which does not adjust the refractive index, the optical path switching takes place by reflecting all of the lights on a side wall of the groove. If the groove is filled with a liquid which adjusts the refractive index, the light is guided through the groove.
There has been proposed a method for switching optical paths by pouring a refractive index-matching liquid (i.e., a liquid having the same refractive index as the core of an optical waveguide) into a groove formed in a crossing point of optical waveguides arranged in a lattice pattern (Meeting of IEICE, C-191, 1992).
Likewise, there is another proposal for switching optical paths, where the method includes the steps of generating bubbles by an electrolytic action to remove a liquid in a groove, or disappearing the bubbles by recombination using catalytic electrodes to refill the liquid in the groove (U.S. Pat. No. 4,988,157) .
According to the above two proposals, the groove filled with the refractive index-matching liquid opens to the outside.
As an example of a sealable structure for switching optical waveguides has been proposed one which uses electro capillarity (Japanese Patent Application Laying-open No. 6-175052 (1994); Spring meeting of IEICE, B-807, 1993; IEICE Trans. Com. Vol. E77, No. 2 February 1994). The proposed method includes the steps of supplying mercury and electrolyte in a tube placed at a crossing point of two optical waveguides with different optical axes and switching optical paths in response to the conditions in which a light passes through the tribe or it is reflected by the mercury in the tube. The switching of optical paths sets by the movement of mercury through the application of a voltage between the two electrodes attached to the respective ends of the tube.
Furthermore, Japanese Patent Application Laying-open No. 7-92405 (1995) discloses an electro-capillarity optical switch with a slit in which mercury is moved by an electric capillarity phenomenon. For smoothing the movement of mercury, the optical switch is provided with a groove as an alternative path or bypass through which the electrolyte in front of the mercury is moved to the back of the mercury when the mercury is driven in the slit.
The optical switch disclosed in U.S. Pat. No. 4,789,228 includes two separated prisms facing each other to form a closed space therebetween where two different dielectric liquids are contained without forming an intimate mixture. One of the liquids is responsible for causing the total reflection of light and the other is responsible for causing the transmnission of light. Therefore, the switching of optical paths is performed by converging the higher dielectric constant of the two dielectric liquids on a predetermined position in the closed space through the application of electric field.
There is another type of optical switch in which the movement of liquid to be used for switching optical paths is performed by heating the liquid through a heater arranged near the groove. For example, an optical switch in the type of optical waveguide disclosed in Japanese Patent Application Laying-open No. 5-88030 (1993) performs the movement of liquid as follows. That is, the optical switch comprises an optical waveguide formed on a substrate and a gap in the form of a groove which is formed so as to cross a core of the waveguide, where the gap stores a refractive index-matching liquid having the same refractive index as that of the core. A means for regulating the temperature of liquid (e.g., thermoelectric cooling element) is arranged in proximity to the gap. This element forcefully heats and vaporizes the liquid in the gap or forcefully cools and condenses the liquid.
The optical switch of the type of self-holding disclosed in Japanese Patent Application Laying-open No. 5-249388 (1993) comprises a decompressed space that cross a crossing portion of two optical waveguides that crosses each other and heaters arranged respectively on both ends of the decompressed space.
The waveguide optical switch disclosed in Japanese Patent Application Laying-open No. 8-94866, includes a substrate having deposited thereon a clad formed with a slit which is provided at each end thereof with a liquid reservoir filled with a refractive index-matching liquid. This buffer structure buffers the shock generated when the refractive index-matching liquid moves in the slit to thereby prevent the occurrence of cavitation. Further, the above-described optical switch is provided with at least one groove so as to encircle the slit positioned in a surface of the substrate which surface is to be bonded to a topical cap for sealing the clad formed with the slit.
Furthermore, an optical switch of the type of controlling a surface tension disclosed in Japanese Patent Application Laying-open No. 8-62645 (1996) (Autumn Meeting of IEICE, B-923, 1994) comprises a slit that crosses a crossing portion of optical waveguides, the right volume of a refractive index-matching liquid being poured in the slit, a thermal heater that separates and heats several portions in the slit to provide a gradient of surface tension of the index-matching liquid in the longitudinal direction of the silt. The gradient of surface tension causes the above liquid to move in the slit. A narrowed portion may be formed in the middle of the slit. Also, an alternative path (i.e., a bypass slit) may be formed in the side of a lid substrate or on the side of the optical waveguide substrate.
FIG. 1 shows an optical switch structure according to the previously proposal described technique in Japanese Patent Application Laying-open No. 9-133932 (1997) entitled "Optical Switch". In FIG. 1, the reference numerals 101i, 101o, 102i, and 102o denote optical waveguides, 103 denotes a groove in which a refractive index-matching liquid is driven, 104 denotes a refractive index-matching liquid, 105 denotes a groove into which an adhesive is poured (depressed part), and 106 denotes a groove (extruded part) into which an adhesive is poured, 107 denotes an adhesive, 108 denotes an optical waveguide substrate, and 109 denotes an upper substrate.
As shown in FIG. 1, optical waveguides 101i, 101o, and optical waveguides 102i, 102o perpendicular to the former are formed on a surface layer portion of the optical waveguide substrate 108.
In the crossing point of these optical waveguides, a groove 103 is formed in which a refractive index-matching liquid is driven. The groove 103 has a depth enough to block each of these optical waveguides, and also it has a wall surface being inclined so as to reflect an optical signal of the optical waveguide 101i to the optical waveguide 102o. Furthermore, the refractive index-matching liquid 104 in the groove 103 is sealed by covering with a glass plate 109 provided as an upper substrate. On both ends of the groove, heaters are arranged but not shown in the figure because of simplifying the illustration.
In the manufacturing method as described above, the refractive index-matching liquid 104 is poured into the groove 103 and then a glass plate 109 is fixed by an adhesive 107 so as to close an opening of the groove 103 from above.
However, the conventional techniques proposed for optical switches and the above attempts have some disadvantages to be solved with respect to their quality and reliability.
In the open system described above, for instance, there is apprehension that the switching characteristics of the optical switch deteriorate as a result of the evaporation of index-matching liquid, the mixing of dust or the like into the above liquid, or the like. Perfect sealing avoids such disadvantage but such measure cannot be applied to the present invention because of the operating principles of the optical switch.
If the optical switch is of the type which allows perfect sealing, it is very difficult to pour the required amount of the index-matching liquid into the groove and to put the lid on the waveguide substrate and seal off the gap or clearance between the lid and the substrate. The reasons for this are as follows. Firstly, the volume of the groove is on the order of picoliters (pl), so that there is no appropriate means for measuring the volume of the liquid to be injected and nor means for injecting the liquid. Secondly, a glass to be provided as a raw material of the optical waveguide substrate is well wetted and thus the liquid poured into the groove is immediately spread outwardly, so that it cannot be stopped at a desired portion. Thirdly, the liquid that spreads over the glass substrate inhibits wetting of the glass and the adhesive to be used for covering the groove with the lid. Consequently, a stable bonding has not been attained.
Further, when fabrication of matrix type optical switches is contemplated, it is very difficult to perform metering and injecting the refractive index-matching liquid for the groove of each switch and also pouring the adhesive in the groove of each switch.
In the case of the conventional technique illustrated in FIG. 1, the volume of the index-matching liquid 104 to be poured into the groove 103 is extremely small. Dimensions of the groove 103 are approximately 10 .mu.m in width, 40 .mu.m in depth, and 100 .mu.m in length, so that the groove 103 receives the volume of the liquid on the order of several ten pico liters (pl). It is very difficult to measure and pour such small volume of the liquid.
Covering the lid with a groove 103 by using the adhesive 107 causes unstable operation of switching optical paths. In addition, there is the possibility that the refractive index-matching liquid 104 changes in its transparency and refractive index when the liquid contacts the adhesive 107 for a protracted period of time. Consequently, the optical characteristics of the optical switch may he deteriorated. Furthermore, it the sealing becomes incomplete as a result of deterioration of the mechanical strength of the adhesive 107, the index-matching liquid escapes to the outside. Consequently, the optical switch does not perform the switching operation any longer.
Furthermore, it is difficult to cover the groove 103 filled with the refractive index-matching liquid with the glass plate 109 using the adhesive 107. In general, a silicone oil is used as a refractive index-matching liquid. However, the adhesion properties of the adhesive becomes poor when a silicone oil is present on a surface of the glass that is provided as a raw material of the optical waveguide substrate. The wettability between the silicone oil and the glass is very high, so that it is difficult to prevent the silicone oil from spreading and wetting over the substrate's surface when the silicone oil is poured in the groove. Thus, the sealing with high reliability cannot be attained in the case of fixing the lid through the adhesive 107. Furthermore, a gap is formed between the substrate 108 and the glass plate 109 when the groove 103 is covered with the lid. The gap is narrower than a width of the groove, so that the refractive index-matching liquid 104 tends to escape from the groove 103 to the gap between the substrate 108 and the glass plate 109 by an effect of the capillarity. Therefore, the sealing cannot be completed because of the same reasons as described above.